Many ways have been disclosed for coupling DC power and differential signals to a twisted wire pair. The DC power is used to power equipment connected to the other end of the wire pair, while the differential data allows the equipment at both ends of the wire pair to communicate with each other. Such a system is sometimes referred to as a Power over Data Lines (PoDL) system.
The various termination components should present a low differential data insertion loss, a high common mode noise insertion loss, and a high differential mode return loss (incident power/reflected power). US Publication 2018/0026525, assigned to the present assignee, describes various termination techniques and can provide more background information. FIG. 1 is reproduced from US Publication 2018/0026525.
In FIG. 1, a media dependent interface (MDI) connector 160 is coupled to a twisted wire pair (not shown) carrying both differential data and a DC voltage. The coupling for both ends of the PoDL system may be identical, except that the Powered Device side does not have its own power supply.
A PHY 150 outputs differential data and receives differential data via the MDI connector 160, a common mode choke (CMC) 210, and AC coupling capacitors C1 and C2. The CMC 210 attenuates common mode RF noise from the wire pair. PHY 150 represents the physical layer in the OSI model and is a transceiver that typically includes signal conditioning and decoding circuitry for presenting bits to the next stage. The term PHY is a term of art and is defined by various IEEE standards, depending on the particular application. The PHY is typically an integrated circuit. A digital processor (not shown) is coupled to the PHY 150 for processing the data.
Since the circuit of FIG. 1 is on the Power Sourcing Equipment (PSE) side, DC voltage from a power source 140 is coupled to the wires by separate inductors 142. The inductors 142 block AC and pass DC.
The wires from the MDI connector 160 are terminated by resistors R1 and R2 and capacitors C3 and C4 to minimize reflections.
In a PoDL system, the PHY must be protected from the DC power. Further, common mode RF noise coupled to the wire pair must be attenuated so as not to interfere with the detection of the differential data. It is sometimes desirable to provide DC isolation of the PHY using a transformer to protect the PHY against DC shorts and other conditions, but this adds an additional component and increases size and cost. It is desirable to provide such features without undue loading that would adversely affect the signal integrity of the differential data. It is also desirable to minimize the component count for cost and size.
What is needed is an improved termination circuit in a PoDL system employing DC isolation of the PHY using a transformer, where the circuit has a low-component count for reducing size and cost, low loading on the PHY, low differential data insertion loss, high common mode noise insertion loss, and high differential mode return loss.